Preparation of 1-carvone



United States Patent Ofilice 2,802,874 Patented Aug. 13, 1957 PREPARATION OF l-CARVONE Robert H. Ritstna, Kalamazoo, Mich, assignor to A. M. Todd Company, Kalamazoo, Mich, a corporalion of Michigan No Drawing. Application January 13, 1954, Serial No. 403,930

6 Claims. (Cl. 260 587) This invention relates to the preparation of l-carvone, particularly to a method for the hydrolysis of l-carvoxime to produce l-carvone in high yield and in a state of high purity.

The importance of l-carv'one in the flavoring and perfume industries is well known. It is a significant or a major component of many oils isolated commercially from plants. It constitutes about 65 percent of natural oil of spearmint and is largely responsible for the odor and flavor of this oil.

l-Carvone has not, until fairly recently, been produced synthetically to any appreciable extent, both because of the lack of an economical method and because of the lack of an industrial source of a suitable starting material. It has been known for many years that 'l-carvone could be produced by the hydrolysis of l-carvoxime, but the process has been characterized by low yields of a satisfactory grade of l-carvone. Furthermore, it is only fairly recently that a convenient source of l-carvoxime has been developed. This latter diflicul'ty has now been overcome to a large extent by the recent availability in commercial quantities of d-lirnonene isolated from citrus oils, especially from orange oil. l-Carvoxime can be preparedreadily from d-limonene by first reacting the d-limonene with nitrosyl chloride to form d-limonenc nitrosochloride and subsequently dehydrohalogenating the nitrosochloride to form l-carboxime. This procedure offers the possibility of producing l-carvone economically, provided the hydrolysis of l-carboxime can be carried out so as to give a high yield of l-carvone in a state of high purity. It is generally recognized that to be of commercial value the l-carvone so produced must have a specific rotation of at least -60 degrees, or better.

As mentioned previously, l-carvoxime has been hydrolyzed to form l-c'arvone, but the heretofore available procedures for carrying out the hydrolysis leave much to be desired with respect to the production of a high yield of. high purity product. In one representative conventional procedure the hydrolysis of l-carvoxime was carried out by passing steam through a mixture of the oxime and a dilute acid, maintained at a pH of about 0.7, until no further oil passed over with the steam. Using this procedure, crude l-carvone was separated as an oily layer from the distillate in a yield of about 66 percent, based on the l-carvoxirne employed. However, the oily product had a specific rotation of only 57 degrees and was unsuitable for use without further purification, e. g. by fractional distillation.

It has been shown that the yield and purity of l-carvone obtained by the acid hydrolysis of l-carvoxime are influenced profoundly by the pH of the solution in which the hydrolysis is carried out. Thus, when the pH of the mixture in the procedure just described was maintained at 1.0, the yield of crude product was increased to about 72 percent, but the specific rotation of the crude prodnot was only about 53 degrees. The maintenance of the hydrolysis mixture at a pH much below 0.7 or much above 1.0 leads to unsatisfactory results of other sorts. At a pH of 0.5, or below, the odor of the product has been shown to be unsatisfactory using conventional procedures and at a pH much above 1.9 the product is contaminated with an undesirable proportion of the oxime, possibly because of its lower rate of hydrolysis at the higher pH and the consequent distillation of a portion of it along with the l-carvone.

As a compromise, it has been recommended that the crime and aqueous acid be mixed and the process carried out so as to maintain the reaction mixture at a pH of about 0.8 to about 1.0 during distillation with steam. Under these conditions, a yield of 65 percent of crude product is obtained having a specific rotation of about 50 degrees. Fractional distillation of the crude product gives a yield of about 54 percent, based on the oximc, of a product having a specific rotation of 60 degrees. It is apparent that this leaves much to be desired as a manufacturing process and that an improved process for the hydrolysis of l-carvoxime which would make possible the production of high purity l-carvone in a substantially greater yield than that heretofore attained would be of great value.

It has now been found that, by following a particular procedure herein first described, l-carvoxime can be bydrolyzed readily with aqueous acid with the production of l-carvone as obtained directly from the reaction mixture by steam distillation having a specific rotation of about 60 degrees, or better, without further purification other than separation and drying. Furthermore, the yield of product obtained is generally at least about percent, or more, of the theoretical amount, based on the l-carvoxime used. The new procedure thus provides a method for the production of l-carvone from l-carvoxime whereby the amount of high quality l-carvonc obtained from a given amount of l-carvoxlme is at least 50 percent greater than the amount obtainable by previously known procedures.

The new procedure is based upon the discovery that, by a proper balancing of the reaction conditions, the conversion of both l-carvoximc and l-carvone to undesirable products, e. g. by racemization or by decomposition, is suppressed, but that, at the same time, the desired hydrolysis is not interfered with seriously. An essential feature of the new procedure is the addition of the Lcarvoxime to a boiling aqueous acid solution as the hydrolytic agent at a rate approximately the same, on a molar basis, as that at which the l-carvone is removed from the mixture by distillation with steam. Under the conditions of pH maintained in the hydrolysis mixture, it appears that the oximc is hydrolyzed almost instantly to l-carvone. There is thus present at any time in the hydrolysis mixture only a very low concentration of either l-carvoximc or of l-carvoue and the tendency for these products to be raccrnized or to be converted in other Ways to undesirable products at the hydrolysis temperature and under the influence of the aqueous acid is overcome to an extent greater than in any procedure heretofore described.

An additional preferred feature of the process is the continuous return to the hydrolysis mixture of the aqueous portion of the distillate. in this manner, the distilled l-carvone is continuously washed with water and the small amount of I-carvoxime which distills from the hydrolysis vessel along with the l-carvone and steam, and which would otherwise appear as an impurity in the crude lcarvone, is continuously washed from the l-carvone layer and returned to the hydrolysis vessel. Not only does this preferred procedure give rise to a higher quality of the l-carvone collected in the receiver, but it also increases the yield of l-carvone, based on the amount of l-carvoxime charged into the hydrolysis vessel. When the procedure of the invention is followed, it is possible and practical to obtain a yield of at least about 80 percent, generally about 83 percent, of l-carvone, based upon the l-carvoxime charged into the hydrolysis vessel, the product having a specific rotation of -60 degrees, or better, when a good grade of l-carvoxime is used, without purification other than to separate it as an oily layer from the steam distillate and dry it. For most purposes the product is adequately pure without further purification procedures, although it can be purified further by conventional procedures if neecssary or desirable.

The hydrolysis of l-carvoxime according to the new procedure is carried out by adding the oxirne gradually to a boiling aqueous acid solution, having a pH of approximately 0.5 to 1.0, which is contained in a conventional vessel suited for distillation with steam and fitted with a condenser and receiver. Distillation with steam is continued at a rate sufficient to remove the l-carvone substantially as fast as it is formed, the l-carvoxime being added at substantially an equivalent rate. Inasmuch as acid is consumed during the hydrolysis, e. g. in the formation of hydroxylamine salt, additional quantities of acid can be added continuously or from time to time, if necessary, to maintain the pH of the hydrolysis mixture within the range of values given. It is to be noted that, employing the new process, the pH of the acid solution need not be controlled within a range as narrow as is the case using hitherto known procedures. This is of a great deal of advantage because of the known dilficulty of determining and controlling accurately the pH of boiling aqueous acid solutions.

The actual rate of addition of l-carvoxime will, of course, depend to a considerable extent upon the rate of distillation. One convenient way of judging the correctness of the rate of addition of the oxime is to observe the ratio of l-carvone to water in the distillate and to regulate the rate of addition of oxime to maintain the ratio somewhat less than its possible maximum under the conditions of temperature and pressure of the distilling operation. Generally speaking, l-carvone should cease distilling with the steam within a few minutes if the addition of l-carvoxime is arrested. Other criteria for regulating the rate of addition of the oxime will be apparent and the invention is not limited with respect thereto.

It is thus apparent that the procedure is actually in the nature of a continuous process and that it need be interrupted and the hydrolysis vessel dumped and recharged with fresh aqueous acid only after the concentration of hydroxylamine salt, or of other products which may he formed, has built up in the hot solution to a point Where the desired hydrolysis is affected adversely. The frequency with which this must be done has not been determined.

The vapors evolved from the boiling hydrolysis mixture are condensed to a liquid which separates in the receiver into an aqueous layer and an oily layer. The oily layer consists principally of pure l-carvone together with a very small proportion of dissolved l-carvoxirne, and the aqueous layer consists almost entirely of water having dissolved therein a major proportion of such of the l-carvoxime as distills with the steam. The aqueous layer is also saturated with l'carvone although the actual concentration of l-carvone is low. The aqueous layer is generally returned from the receiver to the hydrolysis vessel, preferably continuously. Substantially all of the distilled l-carvoxime is thus returned to the hydrolysis vessel where it is again subjected to the hydrolytic environment. The l-carvone dissolved in the aqueous layer is also returned to the hydrolysis vessel and thus re covered. Although it is not usually necessary, it may sometimes be advisable to conduct the condensate from the condenser to the receiver or other layer-separation vessel in such a way that the aqueous and oily layers are agitated together with some degree of violence to promote the best possible extraction of the l-carvoxirne from the oily layer by the aqueous layer. Generally speaking, however, adequate extraction of the oily phase by the aqueous phase occurs during the condensation procedure.

The addition of l-carvoxirne to the hydrolysis vessel is eventually stopped and, after a little further distillation of the liquid therein to insure completion of hydrolysis of the most recently added lcarvoxime, and dstillation of the last portion of l-carvone, the hydrolysis vessel is discharged to waste. It can then be recharged with fresh aqueous acid and the procedure continued.

The l-carvoxime can be added to the hydrolysis vessel in any convenient way. Thus, it can be added as small charges of the molten or crystalline product or the crystals can be slurried with the aqueous distillate layer being returned from the receiver to the hydrolysis vessel and a slurry formed which can be pumped into the hydrolysis vessel. Alternatively, the l-carvoxime can be added as a solution, e. g. in an alcohol. It is frequently advantageous to employ crude l-carvoxime including impurities present or formed during its preparation and in this way losses occurring during its purification can be avoided, it being understood, of course, that the quality of the l-carvone obtained in such instances may not be as good, without further purification as that obtained when highly purified l-carvoxime is employed. Other convenient ways of adding the l-carvoxime to the hydrolysis vessel substantially continuously will be apparent. It is convenient, and usually desirable, to introduce the l-carvoxime into the hydrolysis vessel below the level of the liquid therein to minimize as much as possible the volatilization of oxime with the steam.

The acid used in effecting the hydrolysis can be any one of a wide range of acids, including the hydrogen halides, sulfuric acid, phosphoric acid, oxalic acid and other organic and inorganic acids. Mixtures of acids can be used, if desired. The acid is preferably chosen so that it is substantially non-volatile from the solution with steam and, for economic reasons so that the requisite pH of about 0.5 to about 1.0 can be maintained without the use of an unduly high concentration of the acid. For these reasons, the preferred acids are the highly ionized acids which are substantially non-volatile with steam at the concentrations employed, such as sulfuric, phosphoric and oxalic acids. Thus, a 5 percent aqueous solution of oxalic acid is suitable, as is also an approximately 2 percent aqueous solution of sulfuric acid. From a practical standpoint, the acid used is generally phosphoric acid or sulfuric acid, usually the latter, but the invention is not limited as to the particular acid used so long as the requisite pH can be maintained with it in the hydrolysis vessel. The grade of l-carvoxime employed is not critical except insofar as it might contain substances, e. g. d-limonene and the like, which would be likely to distill with the steam and l-carvone and contaminate the latter. In such instances, the l-carvone layer may require a fractional distillation to eliminate such contaminants and the invention contemplates the carrying out of the process under such circumstances.

Certain advantages of the invention are apparent from the following examples, which are given by way of illustration only and are not to be construed as limiting:

Example 1 A dilute boiling solution was prepared consisting of 700 ml. of water and sufiicient 3 N sulfuric acid (about 50 ml.) to adjust the boiling solution to pH 0.8. Twentyfive grams of l-carvoxime was melted and added dropwise below the surface of the boiling solution. The evolved vapors, consisting principally of steam and lcarvone, were condensed and collected as a two-phase liquid in a receiver. The addition of l-carvoxime and the rate of boiling were adjusted so that, when the ad dition of l-carvoxime was interrupted, the distillation of l-carvone with the steam ceased after about five minutes additional boiling. A total period of about 1.25 hours was required for the addition of all of the l-earvoxime.

Dilute sulfuric acid was added at intervals during the above procedure to maintain the pH of the solution at 0.7 to 0.9. A total of 85 ml. of 3 N sulfuric acid was added in this manner. The aqueous phase which collected in the receiver was withdrawn continuously and returned to the boiling solution. After all of the lcarvoxime had been added, boiling was continued for several minutes until no further oil was observed entering the receiver and the process then discontinued.

The oily phase was separated from the mixture in the receiver. It weighed 18.6 grams, was practically colorless, and, after drying, had an observed rotation of 5S.8 degrees corresponding to a specific rotation [al :6l.6 degrees. The yield was 82 percent of the theoretical amount of l-carvone, based upon the l-carvoxime used.

The procedure was repeated except for the addition of the l-carvoxime in crystalline form in small portions directly to the boiling acid solution. Identical results were obtained.

Example 2 The procedure of Example 1 was repeated except that the pH of the solution was maintained between 0.55 and 0.8 during the hydrolysis. The product obtained had an observed rotation of -59.0 degrees and a specific rotation ia] :6l.8 degrees.

Example 3 The procedure of Example I was again repeated except that the pH of the solution was maintained between 0.7 and 0.95 during the hydrolysis. The product obtained had an observed rotation of 59.3 degrees and a specific rotation [txl :62.l degrees. The yield of this product was 75 percent of the theoretical amount, based upon the l-carvoxime used.

By way of comparison, the procedure was repeated except that the pH of the solution was maintained between 0.35 and 0.45. The observed rotation of the product was 4l.6 degrees. When the pH of the solution was allowed to rise to 1.2 during the hydrolysis, the product obtained had a specific rotation [a] =58 degrees.

I claim:

1. In a method wherein l-carvoxime is hydrolyzed with a boiling aqueous acid maintained at a pH of from about 0.5 to about 1.0 and l-earvone is distilled from the hydrolysis mixture with steam and recovered from the steam distillate, the steps which include: regulating the rate of addition of the l-carvoxime to the hydrolysis mixture at substantially the same rate on a molar basis as that at which the l-carvone is distilled therefrom whereby the concentration both of l-carvoxime and of l-carvone in the hydrolysis mixture is kept low; and recovering from the distillate l-carvone having a specific rotation of about degrees in a yield, based on the l-carvoxime added, greater than the yield obtained when the hydrolysis is carried out in identical fashion except for the addition of all of the l-carvoxime to the hydrolysis mixture at one time.

2. In a method for hydrolyzing l-carvoxime to form l-carvone, the steps which include: introducing l-carvoxime into a boiling aqueous acid solution maintained at substantially pH 0.5 to 1.0; removing l-carvone from the boiling mixture by distillation with steam, the l-carvoxime being added at a rate approximately equal on a molar basis to that at which the l-carvone is removed; and adding acid to the boiling mixture to maintain the mixture at about pH 0.5 to 1.0.

3. A method as claimed in claim 2 wherein the steam distillate is condensed and separated into an oily phase and an aqueous phase, the latter being returned to the boiling mixture.

4. The method as claimed in claim 2 wherein the aqueous acid solution is a solution of a highly ionized acid which is substantially non-volatile with steam at a pH of the solution of from about 0.5 to about 1.0.

5. The method which includes: providing a boiling aqueous acid solution at about pH 0.5 to 1.0; adding purified l-carvoxime to the boiling solution to cause its hydrolysis and the formation of l-carvone; distilling the l-carvone from the boiling solution with steam; adding acid to the boiling solution to maintain it at about pH 0.5 to 1.0; regulating the rate of addition of the l-carvoxime to substantially the same rate on a molar basis as that at which the l-carvone is distilled; and condensing the steam distillate to form an aqueous phase and an oily phase, the latter comprising principally l-carvone.

6. The method of claim 5 wherein the oily layer is separated and dried to yield at least about percent of the theoretical amount of l-carvone derivable from the l-carvoxime used as a product having a specific rotation of about 60 degrees.

References Cited in the file of this patent 

1. IN A METHOD WHEREIN 1-CARVOXIME IS HYDROLYZEDD WITH A BOILING AQUEOUS ACID MAINTAINED AT A PH OF FROMM ABOUT 0.5 TO ABOUT 1.0 AND 1-CARVONE IS DISTILLED FROM THE HYDROLYSIS MIXTURE WITH STEAM AND RECOVERED FROM THE STEAM DISTILLATE, THE STEPS WHICH INCLUDE: REGULATING THE RATE OF ADDITION OF THE 1-CARVOXIME TO THE HYDROLYSIS MIXTURE AT SUBSTANTIALLY THE SAME RATE ON A MOLAR BASIS AS THAT AT WHICH THE 1-CARVONE IS DISTILLED THEREFROM WHEREBY THE CONCENTRATION BOTH OF 1-CARVOXIME AND OF 1-CARVONEE IN THE HYDROLYSIS MIXTURE IS KEPT LOW; AND RECOVERING FROM THE DISTILLATE 1-CARVONE HAVING A SPECIFIC ROTATIONN OF ABOUT -60 DEGREES IN A YIELD, BASED ON THE 1-CARVOXIME ADDED, GREATER THAN THE YIELD OBTAINED WHEN THE HYDROLYSIS IS CARRIED OUT IN IDENTICAL FASHION EXCEPT FOR THE ADDITION OF ALL OF THE 1-CARVOXIME TO THE HYDROLYSIS MIXTURE AT ONE TIME. 